1. Field of the Invention
The invention relates to a protective element for protecting a current consumer against overloading. Resistance elements, known as PTC elements, which have a polymer matrix and a powdered filler of conductive material embedded in said matrix have been known for some time. The resistance of these elements increases abruptly by several orders of magnitude if the temperature of the resistance element reaches a switching temperature. It corresponds to the melting point of the polymer, at which the particles of the filler are separated by the melting of the matrix.
One of the ways in which this effect can be used is for current limitation, in particular for interrupting overload currents. In this case, a resistance element which remains at a temperature in the highly conductive range under a nominal current, but is heated by an overload current to such an extent that it reaches the switching temperature, is connected in series with the current consumer as a protective element.
2. Discussion of Background
It has also already been proposed (J. Mater. Res. 6/1 (1991)) to prevent overheating of the polymer in PTC resistors by providing a further powdered filler which, at a critical temperature above this switching temperature, undergoes a phase transition in which it absorbs heat of transformation, so that further heating of the resistor core is prevented, or at least delayed.
Protective elements in which the trigger characteristic, i.e. the response time as a function of the overload current factor, has a specific form are required for various applications. If the overload current amounts to a certain multiple of a nominal current, the protective element is intended to interrupt the current after a certain time, which is dependent on this factor. This applies in particular to motor protective circuits which are in series with an electric motor and must withstand an increased motor starting current, which is for example up to 5 to 10 times the nominal current, for a certain time, for example 1 to 10 seconds. Subsequently, the limit value at which the motor protective circuit interrupts the current is intended to fall virtually to the nominal current, so that only a small overload current is tolerated over a prolonged period to avoid thermal overloading of the motor.
Such motor protective circuits can currently be realized only by relatively elaborate arrangements of different switching elements connected in series, for example a fuse reacting quickly to short, high levels of overload current, such as those caused by lightning strikes, a switch responding to rather more prolonged, more moderate overload currents, such as short-circuit currents for example, and a thermal relay interrupting low overload currents if they persist.
Accordingly, one object of the invention is to provide a novel protective element which has for such tasks a response time which is suitably dependent on the overload current factor and which is therefore adequate within a relatively simple circuit, preferably alone or in series with just one switch or disconnector, for performing such a protective task.
This object is achieved by the invention in the way set out in the defining part of patent claim 1. While conventional PTC resistors for instance have a trigger characteristic which, if correctly set in the range of short, high overload currents and prolonged, low overload currents, responds too quickly under customary motor starting currents or, conversely, allows the required motor starting currents but responds too slowly under short, high overload currents and, in particular, under low, prolonged overload currents, this can be corrected by the measure according to the invention in that the heating of the resistance element in the range of likely motor starting currents is deliberately delayed and the response time is extended as a result.
The advantages achieved by the invention are, in particular, that it makes it possible to produce protective elements for the protection of sensitive components against overload currents which are of a simple structural design, are reliable and can be produced at relatively low cost. Protective elements according to the invention are particularly suitable as motor protective circuits for electric motors or as components of such circuits.